I'm not a lawyer, but I think the defendants would've been committing legal suicide to even suggest to a jury that the tragedy that befell Terri, her husband, and her devoted family had anything to do with spousal abuse.
I've read interviews with the Schindlers, their attorney, and Terri's friends all saying there is no proof she had an eating disorder. They seem to admit there's some possibility of that, but it isn't a certainty. The malpractice defendants had to have some legal theory about Terri's contributory negligence, or they would've been libel for up to $20 million. There had to be some reason given as to why this seemingly healthy and very young woman's heart suddenly stopped beating. Since Shiavo's attorneys sued on the basis of the undiagnosed potassium imbalance, that's what the defense had to prove they didn't cause. Terri had lost nearly 80 pounds since her marriage and was going to a gyno for diagnosis/treatment of missed menstrual periods, both possible symptoms of an eating disorder. Michael and the Schindlers were unable to testify that Terri did not have an eating disorder. This seems like the best winning argument the defendants could've proferred.
It would've been a very different case if Michael Schiavo had been the target of a criminal investigation. According to what we've read here and there, the ER staff where Terri was taken that night was suspicious enough to call law enforcement. The police questioned Terri's father about Michael, and Terri's father said there was no reason to suspect foul play. Under the circumstances, with a wife and daughter fighting for her life in an ICU, I think many law enforcement agencies would've just closed the book, quietly walked away, and given the family their privacy at such a painful time.